FIELD OF THE INVENTION
The invention relates to a method for packet-oriented data transmission in a digital transmission system, in which an ARQ protocol and channel coding are used for error minimization with coders and decoders which emit soft information for evaluation of a retransmitted data frame, and to an configuration for performing the method.
A method and a configuration of the type mentioned initially are described in IEEE COMMUNICATIONS LETTERS, Volume 1, No. 2, pages 49-51, K. R. Narayanan et al., "A Novel ARQ Technique using the Turbo Coding Principle". According to the method described there, logarithmic likelihood ratios (LLR) for the data to be decoded are determined during an earlier attempt to decode data in a data frame. If a decoding attempt fails, then the corresponding data frame is retransmitted. When the data in the retransmitted data frame are being decoded, the logarithmic likelihood ratios determined during the earlier decoding attempt are used as a-priori information in a forward-progressing manner.
In a conventional transmission path for transmission and for reception of digital data, the data from a data source are combined into blocks and are fed to a convolution coder. The coded data blocks are encrypted in an interleaver and are then transmitted via an ISI channel (ISI=Intersymbol Interference). At the receiver end, the data blocks are equalized in an equalizer. Once the equalizer has removed the interference, the equalized data blocks are processed in a deinterleaver, and this emits the data blocks, after deinterleaving, to a decoder, which then corrects the majority of the errors and passes its output signals to the data sink.
The aim in such a system is to use the channel coding to correct the influence of those errors which occur in the ISI channel during transmission. The coding increases the bit rate, and all the arriving information is subjected to channel coding. Depending on which channel is used, various us coding methods are used, for example different coding versions are used for data transmission than for voice coding. Forward error correction (FEC) may be used for error correction in the physical layer, and error correction with the ARQ protocol may be used for layer 2, and these types of error correction lead to a transparent or nontransparent transmission class, respectively.
When error correction is carried out using the ARQ protocol (ARQ=Automatic Request for Retransmission), error correction is carried out deliberately for the data transmission. The ARQ method based on flow control leads to nontransparent transmission, and is particularly effective in conjunction with FEC. in the GSM system, ARQ is used in the RLP (RLP=Radiolink Protocol).
In ARQ protocols, data frames which, for example, are rejected during transmission are requested once again and are retransmitted. When the data are received once again, information which was generated when these data were previously received can be reused. The principle proposed for this purpose is known from IEEE Transactions on Communications, Volume COM-33, No. 5, May 1985, D. Chase, "Code Combining--A Maximum Likelihood Decoding Approach for Combining an Arbitrary Number of Noisy Packets". In this case, the information is transmitted in packets which are coded with a code having a relatively high code rate, and which are repeated in order to achieve reliable communication, if the redundancy in the code is not sufficient to overcome the channel interference problems. The receiver combines packets that are subject to noise signals, in order to obtain a packet at a code rate which is sufficiently low that even channels having extremely high error rates can be combined reliably. The aim is to combine a minimum number of packets to reduce the code rate and the delay to the minimum that is required to decode a predetermined packet (data frame). This is the classic code combining method, which is to be improved. Diversity methods, for example metric combining, are used.